Motor control system



Nov. 19, 1935. A. UMANSKY-. ICSTOR C ONTROL SYSTEM Filed Oct. 11, 1933 2 Sheets-Sheet 1 NOV. 19, 1935. L A, UMANSKY 2,021,757

I MOTOR CONTROL SYSTEM Filed oct. 11, 1935 2 Sheets-Sheet 2 uw k .mww m e T m n m mm A m .5 e H L U0 b Patented Nov. 19, 1935 UNITED STATES PATENT OFFICE MOTOR CONTROL SYSTEM Leonid A. Umanlky, Schenectady, N. Y.,

assignmto General Electric Company, a corporation of New York Application October 11, 1933, Serial No. 693,138

' :1 Claims. (01. 242-75) More specifically the invention relates to control systems for electric motors, and a more specific object is the provision of means for controll ling the input to the controlled motor in accordance with the rate of change of speed of a control element.

In another of its aspects, the invention relates to motor control systems for apparatus having a ll motor driven element operating on a strip oi material and a motor driven reel for supplying the strip to the operating element or taking up the strip delivered by the operating element, and a further object of the invention is the provision 20 of means for maintaining a substantially constant tension in the strip between the operating element and the reel during acceleration and deceleration of the driving motors as well as when the motors are operating at constant speed.

I In carrying the invention into eflect in one form thereof, the energization of a dynamo-electric machine is varied in accordance with'the speed of a variable speed controlling element and this machine is provided with a substantial amount of 30 .fly-wheel eiIect so that its current varies in accordance with the rate of change of speed of the controlling element and this variable current is utilized to exert a controlling action on the device or object which it is desired to control.

As applied to the control of motors for driving an element operating on a strip of material and the supply or take-up reels, a regulator is provided for maintaining substantially constant current input to the reel motor while operating at 40 constant speed, and means responsive to the rate of change 0! speed of the element driving motor are provided for varying the setting of the regulator in proportion to the rate of change of speed of the element driving motor. In one embodiment the rate of change of speed responsive means for varying the regulator setting takes the form of an additional coil on the regulator for modifying the action of the regular operating coil,

50 together with a dynamo-electric machine connected in circuit with the additional regulator coil, supplied with a voltage varying with the speed of the operating element, and designed with substantial fly-wheel effect so that it draws a cur- 55 rent varying with the rate of change of speed of RElSSUED the element driving motor to vary the setting of the regulator.

In illustrating the invention in one form thereof it is shown as embodied in a motor control sys-- tem for the mill and take-up reel motors of a 5 cold strip steel rolling mill. For a better and more complete understanding of the invention reference should now be had to the following specification and to the accompanying drawings in which Fig. l is a simple, diagrammatical repl0 resentation of an embodiment of the invention and Figs. 2 and 3 are simple diagrammatic representations of modifications thereof.

Referring now to the drawings, a strip of material Ill such for example as cold steel is being fed in the direction of the arrow to a pair of reducing rolls ll between which the gauge of the strip is suitably reduced and the strip emerging from the mils is wound upon a take-up reel i2. The rolls I l are driven by any suitable driving means illustrated in the drawings as an electric motor I! of the direct current type which in turn is supplied from any suitable source such as that represented by the variable voltage generator II, with the armature of which the armature of the motor I! is connected in a, loop circuit when the switch Isis in its closed position. Similarly the reel I2 is driven by a direct current driving motor it which may be supplied from any suitable source but which is preferably supplied from the mill motor supply generator I, when the switch i1 is in its closed position. Both motors l3 and it are shown as direct connected to the machine elements which they drive but in practice a gear drive is usually employed.

The supply generator I 4 is driven at a speed which is preferably substantially constant by any suitable driving means (not shown) such for example as a synchronous motor or an alternating current induction motor. As shown, the generator I4 is provided with a field winding l8 supplied from a suitable constant potential source represented by the plus and minus supply lines 20 and its excitation is adjusted by means of a suitable rheostat 2| included in circuit therewith. As is well imderstood, operation of the rheostat 2| to vary the excitation of the field l8 produces a corresponding variation in the voltage of the generator I and the speed of the driving motors l3 and I6 supplied therefrom. The connection of a driving motor in a loop circuit with a variable voltage generator, such as shown in the drawings, is usually referred to in the art as a Ward-Leonard connection and it has the advantage that the speed of the motor can be smoothly and economically varied over a "wide range. The

mill motor 1 3 is provided with a shunt field windenergized from a suitable constant potensource indicated by the plus and minus suplines, which source may be and preferably is same as that indicated by the supply lines 20. ly, the reel motor i8 is provided with a .5 field winding 23 supplied from a suitable i constant potential indicated by plus minus signs and likewise this source is prefthe same as that represented by the supthe diameter of the coil of strip on the reel lz-uilds up, the speed of the reel motor should he gradually decreased. This is accomplished automatically by means of a resistance 24 included in the circuit with the field winding 2 3 of the reel inotor i5, under the control of a. suitable constant current regulator 25. Although this constant current regulator may be of any wellknown suitable type, it is shown as a vibratory contact type regulator having an operating coil 25 energized by the voltage drop across the series resistance 27 so that the regulator is responsive to the armature current of the reel motor IS. A variable resistance 28 is included in the circuit or" the coil 28 for adjusting the current setting of the regulator.

The coil 25 acts against the spring 29 and tends to close the contacts 30. If the mill is a reversing mill and the motor I6 is to operate alternately as a reel driving motor during one pass and as a drag generator on the reverse pass, the regulator is additionally provided with back contacts 3th for controlling the regulating resistance 24 so that the regulator responds to generator action. A switch 3| is also provided for rendering one of the pairs of contacts 39, 30. active depending upon the direction of mill operation. The regulator 25 functions in a well-understood manner to maintain substantially constant cur rent input to the reel motor I6. During the pass illustrated, in which the motor i6 is driving the reel to wind the strip, the movable contact of the switch 3| is moved into engagement with its upper stationary contact to render the regulator contacts 30 active so that the regulator responds and controls the motoring action Similarly, during the reverse pass, when the motor is to act as a drag generator, the movable contact of the switch contact 3| is provided to engage the lower stationary contacts to render the regulator contacts 305 active and to cause the regulator to respond to generator action. Briefly, the operation of this regulator is as follows:

As the coil diameter on the reel I 2 increases during the winding operation, the regulator suitably strengthens the field of the reel motor thereby decreasing the speed of the motor just sumciently to maintain the current input to the motor substantially constant. As is well understood, for any given armature voltage of the motor, this corresponds to constant tension in the strip. In the case of a reversing mill, the strip is unwound from the reel during the reverse pass and drives the motor l6 as a generator at a speed that increases as the coil diameter decreases. Under this conditionfthe regulator suitably weakens the field to maintain the current output and back tension in the strip substantially constant.

It will be seen that if during the starting of the mill and reel from rest, the strip tension is maintained at the same value as that at which it is maintained during operation of constant speed, the reel motor requires more current in order to accelerate the mass of its own armature and that of the reel. Since the regulator 25 tends to maintain the current input to the reel motor I6 substantially constant it prevents the reel motor from taking the required additional current and in so doing it either results in an undesirable decrease in the strip tension or in extreme cases theacceleration of the reel is retarded by such an amount that the strip slackens between the reel [2 and the mill rolls II with the result that the strip tension is lost entirely.

In order to prevent this decrease or loss of tension in the strip during a change in speed of the mill, means are provided for varying the current setting of the regulator in accordance II with the rate of change of speed of the mill. This means is illustrated as comprising a tachometer generator 32 driven by the mill motor l2, an auxiliary dynamo electric machine 33 supplied from the tachometer generator 32 and prol0 vided with a flywheel 34, and an auxiliary coil 35 on the regulator for modifying the operation of the operating coil 26. The tachometer generator 32 is provided with a field winding supplied from a suitable constant potential source 28 indicated by plus and minus signs and thus the voltage of the generator 32 is a measure of the speed of the mill and thus the shaft of the mill motor. Similarly, the fly-wheel motor 33 is pr vided with is. separately excited field winding :1 connected to a suitable constant potential source represented by plus and minus signs. The armature of the ily-wheel motor 33 is connected to the armature of the pilot generator 32 by means of conductors 38 and the armature circuit 01 these two machines is led through the auxiliary coil of the regulator. Preferably the fly-wheel motor 33 is equipped with ball-bearings so that its running light losses are practically negligible.

The auxiliary coil 35 of the regulator is arranged so that it acts in opposition to the operating coil 28 during the period of acceleration of the reel and thus tends to decrease the magnetic pull of the operating coil. It is also obvious that during the deceleration period the flywheel motor 33 acts as generator, pumping the energy back into the generator 32; therefore the direction of current in the coil 35 will reverse, and the latter will then act cumulatively with the coil 26 thereby reducing the eflective setting of the regulator, and causing the reel motor ll to draw less current.

When the mill is operating at a substantially constant speed, the fly-wheel motor 33 draws a very small current due to its small running light losses. However, if the speed of the mill changes, a. larger current flows in the armature circuit of the fly-wheel motor. For any given size of the fly-wheel and excitation of the fly-wheel motor 33, the current flowing through the armature 0f the fly-wheel motor is fairly proportional to the rate of change of speed of the mill. Several factors must be considered however in choosing the proper size of fly-wheel for the fly-wheel motor. The smaller the size of the fly-wheel the more accurately the current of the fly-wheel motor will represent the instantaneous rate of change of speed of the mill. On the other hand, the larger the fly-wheel the greater will be the 7 magnitude of the current drawn by the fly-wheel motor. Also, the greater the rates of acceleration or deceleration of the mill, the smaller should be the size of the fly-wheel in order to obtain the some current in the armature circuit of the fly- 7g thus it becomes necessary to change the eflect is acting differentially with respect to the operoil the auxiliary coil 36 in proportion. Forthis purpose a rheostat is provided which shunts a larger or smaller proportion 01' the current drawn by the fly-wheel motor 33. As shown, this rheostat is mechanically coupled to the regulator setting rheostat 23 and is operated by a common hand wheel so that it varies the eirect of the auxiliary coil 35 in proportion to the amount the regulator setting rheostat 23 varies the eiiect o! the coil 28 during acceleration and deceleration oi the reel.

The operation of the system is as follows:

Assuming that the supply generator I is being driven at its proper speed, the mill motor l3 and the reel motor I I are started from rest by operating the rheostat 2| to decrease the resistance in the field circuit of the supply generator and thus to increase the generator voltage. For the pass in which the motor l6 driving the reel to wind the delivered strip, the movable contact of switch 3| is moved to engage its upper stationary contact, thereby rendering the regulator contacts 30 active so that the regulator responds to reg ulate the motoring action. a For any given mill speed, the fiy-wheel motor 33 operates at a corresponding speed and draws an almost insignificant current from the tachometer generator 32.

As long as the mill speed remains constant, the regulator 25 functions in the previously described manner to maintain substantially constant current input to the reel motor It by increasing the excitation of the reel motor to decrease the speed thereof as the diameter of the coil on the reel l2 increases. Thus, for any mill speed the regulator 25 maintains substantially constant tension in the strip between the reel and the mill rolls.

If now the speed of the mill is increased the voltage of the pilot generator 32 .will increase in proportion, and the fly-wheel motor 33, tending to follow the speed of the pilot generator 32 will draw an increased current in order to accelerate its armature and the fiy-wheel 34. This current is approximately proportional to the rate of change of speed of the pilot generator 32 and is thus approximately proportional to the rate oi! change of speed of the mill motor l3 which in the illustrated system serves as the variable speed controlling element. Thus the value of the current drawn by the ily-wheel motor serves as a measure or the rate of change of speed of the mill. Since this current flows through the auxiliary coil 35 which at this time ating coil 26, the eilect is automatically-to increase the setting" of the regu ator without tampering with its operating coil 26 or its circuit. The greater the acceleration of the mill, the greater will be the bucking action of the auxiliary coil 35 and thus the higher will be setting of the regulator. In other words, the increased current flowing in the auxiliary coil 35 decreases the efiect oi the operating coil 26 and this allows the reel motor it to draw the increased current that is necessary in order to accelerate its own armature and the mass of the reel without decreasing or losing the tension in the strip between the reel and the mill rolls. Thus, it will be seen that the setting of the regulator is automatically and prop rly adjusted to allow the reel motor to draw the current required 5 for acceleration. As the illy-wheel motor comes up to speed, the acceleration gradually decreases and thus the regulator 28 gradually returns to its original setting. I

. When the mill is decelerated by decreasing the 10 voltage 01' the supply generator I, the direction of the current flowing in the circuit oi the ilywheel motor and the auxiliary coil 35 will reverse. This is due to the fact that the fly-wheel is an energy storage device and gives up its stored 15 energy in the form of electric current when the voltageot the pilot generator is decreased below the value of the counter voltage of the fly-wheel motor 83. As a result the eilect of the auxiliary coil 35 and the operating coil becomes cumulative so thereby decreasing the eiiective setting or the regulator, which condition is exactly what is required for decelerating the reel motor so as not to rupture the strip or unduly increase its tension.

It will thus be seen that substantially constant tension is maintained in the strip between the mill rolls and the reel not only duringconstant speed operation of the mill but during periods of acceleration and deceleration as well.

In the above description it has been assumed that the reel i2 is operating as a take-up reel. However, the mill is a reversing type mill, during the succeeding pass, the reel operates as a supply reel, and the motor it operates as a drag 35 generator driven by the strip, returning energy to the system and maintaining back tension in the strip between the reel and the mill. For this operation, the movable contact of the switch 3| is moved into engagement with its stationary 40 contact thereby rendering the back contacts 30- of the regulator active to control the resistance The result of this change is to cause the regulator to respond to "generating" action. In this connection it is to be noted that when the 45 contacts 30 areactive an increase in the armature current causes the contacts 30 to short-circuit the resistance 24 and thereby strengthen the field. This is the correct regulating action if the regulated machine is operating as a motor. 50 If the contacts 30a are active, an increase in the armature current causes the contacts 30. to open, insert the resistance 24 in the field circuit and thereby weaken the field, which is the. correct regulating action if the regulated machine is 65 operating as a generator. The operation of the fly-wheel dynamo-electric machine 33 under this condition is similar, but the reverse of that Just described for the preceding pass. Thus the terms take-up reel and motor" are to be un- 60 derstood as employed in a descriptive and not in a limiting sense.

The arrangement shown in the modification of Fig. 2 is in most respects identical with the system of Fig. 1. However, in this modification 65 a variable rheostat M is included in the field circuit of the mill motor 45 so that the mill speed is controlled not only by the generator voltage, but also by weakening the field of the mill motor. As shown, the motor field rheostat I4 is mechani- 70 cally connected to the rheostat 46 in the field circuit of the supply generator 41. The two rheostats are operated by a common hand-wheel 48. The resistance sections and movable contacts of these rheostats are so arranged that the speed Tl not supplied from a pilot generator as in the system of Fig. 1 but on the other hand is connected directly across the mains of the supply generator 47. The armature of the fly-wheel motor is connected in the circuit of the auxiliary coil SI of the regulator 52 which is in all respects identical with the regulator 25 of Fig. l. The separately excited field winding 53 of the fiy-wheel motor 56 and the field winding 54 of the reel motor 55 are connected to suitable constant potential sources indicated by plus and minus signs as in the system of Fig. 1.

There is a definite advantage in connecting the armature of the fiy-wheel motor 50 across the terminals oi the supply generator instead of energizing it from a separate pilot generator as in the system of Fig. 1. It is obvious that the generator voltage increases before the mill speed can follow the increase of voltage. Therefore, the regulator forcing coil will be energized slightly earlier than would be the case if the fiy-whee motor were connected to a pilot machine. This anticipating action is especially advantageous when starting up from rest.

After the mill and reel have been accelerated by raising the generator voltage, the rolling speed may be further increased by weakening the field of the mill motor 45. Obviously, any suitable means, such for example as a rheostat similar to the rheostat 44 and actuated by the handwheel 48 may be utilized to weaken the field of the fiy-wheel motor 53 simultaneously with the weakening of the field of the mill motor. This will cause the fly-wheel machine to draw a suitable accelerating current from the generator mains; this current, flowing through the forcing coil 5| will suitably increase the setting of the regulator and will permit the reel motor to maintain the strip tension substantially constant even during this part of the accelerating period.

The operation of the modified system of Fig. 2 is otherwise substantially identical with the previously described operation for the system of Fig. l and repetition is unnecessary.

In the modified system shown in Fig. 3 the fiyi wheel motor 60 is supplied from a pilot generator 6| driven by the mill motor 62 as in the system of Fig. l. The regulator 63 is in all respects identical with the regulator 25 in Fig. 1. The

K modified system of Fig. 3 is diflerent Irom the system of Fig. 1 in that the armature of a I booster generator 64 is included in the armature circuit of the reel motor 65 between the armature terminals of the latter and the mains of the supply generator 66. The booster generator 64 is driven at a speed which is preferably substantially constant by any suitable driving means illustrated as an alternating current motor 61 supplied from a suitable source indicated by the three supply lines 68. Either a synchronous motor or an induction motor may be utilized to drive the generator 61. As shown, the field winding III of the booster generator is included in' the circuit of the armature of the fiy-wheel motor 60 and the auxiliary regulator coil.

In operation, when the mill is operating at substantially constant speed, the running light current of the fly-wheel motor 60 is practically negligible, with the result that the excitation and generated voltage of the booster generator 64 are practically nil. As the speed of the mill is increased by increasing the voltage of the supply generator 60, the fly-wheel motor 60 draws a current proportional to the rate of change of speed oi the mill. This increased current flowing through the field winding ll oi the booster gen- 5 erator will cause the latter to generate and introduce an additional voltage in the armature circuit of the reel motor 65. This additional voltage introduced in the armature circuit of the reel motor will cause the latter to accelerate with- 10 out any tendency to diminish the strip tension. It is also evident that the polarity of the booster generator is automatically reversed so that it becomes a bucking generator during deceleration and assists the reel motor to decelerate properly without any tendency to increase the tension.

In other words, in the arrangement shown in Fig. 3, the additional input to the reel motor, required during the accelerating period, is obtained either wholly or partly by temporarily increasing the applied voltage, instead of obtaining this additional input entirely by increasing the armature current as in Figs. 1 and 2. In an extreme case, a properly designed booster generator GI, may iully take care of maintaining tension during acceleration and retardation, dispensing entirely with the forcing coil on the regulator 53. The remainder of the arrangement and its operation is in all respects identical with that 01' Fig. l and will not be repeated.

In accordance with the provisions oi the patent statutes this invention is described as embodied in concrete form but it will be understood that the apparatus, elements, and connections shown and described are merely illustrative and that the invention is not limited thereto since alterations and modifications will readily suggest themselves to persons skilled in the art without departing from the true spirit of this invention and the scope of the annexed claims.

What I claim as new and desire to secure by Letters Patent 01 the United States, is:

1. In combination, a variable speed'element, means for supplying a voltage varying with the speed of said element, a dynamo-electric machine connected to said supply-means and provided with substantial fly-wheel effect so that its current varies with the rate of change of speed of said element.

2. In a control system a variable speed element, control means, and means for actuating said control means comprising a dynamo-electric machine provided with substantial fly-wheel eflect and means for supplying a voltage varying with the speed of said element to said machine so that its current varies with the rate of change of speed of said element.

3. A control system comprising a variable speed element, a controlled object, a control device for said object, and means for actuating said device 00 comprising a dynamo-electric machine provided with substantial fiy-wheel effect and means for supplying a voltage varying with the speed of said element to said machine so that its current varies with the rate of change of speed of said element.

4. A control system comprising a variable speed element, a source of supply connected with said element, a dynamo-electric machine supplied from said source and provided with substantial fiy-wheel effect so that its current varies with the rate of change of speed of said element, and a control device responsive to said current.

5. A control system comprising a variable speed element, means for producing a voltage substanchine supplied from said generator and provided with substantial fly-wheel eflect so that its current varies with the rate of change of speed of said element, and means responsive to said current for controlling said motor.

7. A control system for an electric motor comprising a variable speed element, a regulator for maintaining an operating characteristic of said motor substantially constant, means for producing a voltage varying with the speed of said element and a dynamo-electric machine energized from said voltage producing means and provided with substantial fly-wheel eflect so that its current varies with the rate of change of speed of said element, and means responsive to said current for controlling the action of said regulator.

8. A control system comprising a dynamoelectric machine, supply means for said machine, a booster generator connected in circuit with said machine and said supply means, a variable voltage supply source and a dynamo-electric machine supplied from said source and provided with substantial fly-wheel eifect for controlling said generator 9. A motor control system comprising an electric motor, a source of supply therefor, a booster generator connected in circuit with said motor and a dynamo-electric machine provided with substantial fly-wheel eiTect and connected in the iield circuit of said generator so as to control the voltage supplied to said motor.

10. A motor control system comprising a variable voltage source, a plurality of motors supplied from said source, a booster generator having its armature connected in circuit with the armature of one of said motors, means for supplying a voltage varying with thespeedoi' the other of said motors, a dynamo-electric machine supplied from said supply means and having substantial fly-wheel effect and connections from the armature oi said machine to the field circuit of said generator so that said generator controls said motor in accordance with variations in the current of said machine.

11. A system of motor control comprising a plurality of driving motors, a regulator having an element responsive to the current of one of said motors for maintaining the current input to said motor substantially constant, means responsive to rate of change of speed of the other of said motors for controlling the action 01' said regulator comprising a coil on said regulator for modifying the action of said current responsive element, an auxiliary motor energized in accordance with the speed of said other motor, the armature of said auxiliary motor being connected to said coil and.designed to have iiy-wheel effect, and a booster generator having its armature connected to said regulated motor and its iield winding included in circuit with said coil and said auxiliary motor.

12. A control system for apparatus having an element operating on a strip of material and a take-up reel tor the delivered strip comprising a motor for driving said element, a motor for driving said reel, means for holding substantially constant tension in the strip comprising a regulator having annperating element responsive to the current of said reel motor for maintaining substantially constant current input to said motor, means for varying the setting of said 5 regulator to maintain the strip tension substantially constant during adjustments in the speeds of said motors comprising an auxiliary motor energized in accordance with the speed of said element driving motor and designed with fly-wheel 1o eii'ect so as to draw a current varying with the change in speed of said element driving motor and a coil on said regulator for varying the action of said current responsive element connected in circuit with said auxiliary motor, and means 16 for introducing a voltage in the reel motor circuit varying with the change in speed of said element driving motor.

13. A control system for apparatus having an element operating on a strip of material and a 20 reel for the strip comprising a motor for driving said element, a motor connected to said reel, regulating means for said reel motor for maintaining the tension oi. said strip substantially constant, and means for modifying the operation of said regulating means substantially in proportion to rate of change of speed of said element driving motor.

14. A control system for apparatus operating on a strip of material and a reel for the strip comprising a motor for driving said element, a motor connected to said reel, a current regulator for said reel motor for maintaining the tension of said strip substantially constant, and means for varying the current setting of said regulator substantially in proportion to the acceleration and deceleration of said element driving motor.

15. A control system for apparatus having an element operating on a strip of material and a reel for the strip, comprising a motor for driv- 40 ing said element, a motor connected to said reel,

a regulator for controlling the operation of said reel motor, and control means for said regulator comprising a dynamo electric machine controlled in accordance with an operating condition or said element driving motor and an electroresponsive device connected in circuit with said dynamo electric machine for modifying the operation of said regulator.

16. A control system for apparatus having an element operating on a strip of material and a reel for the delivered strip comprising a motor for driving said element, a second motor for driving said reel, a regulator for maintaining the current input to said reel motor substantially constant, and means for varying the value of current to be held by said regulator comprising a dynamo electric machine responsive to the rate of change oi! speed of said element driving motor and an electroresponsive device connected in cir- B0 cult with said machine for modifying the action of said regulator.

17. A control system for apparatus having an element operating on a strip of material and a reel for the strip comprising a motor for driving said element, a motor connected to said reel,

a constant current regulator for said reel motor provided with a current responsive element, and means responsive to rate of change of speed 01' said element driving motor for varying the cur- '70 rent setting of said regulator to maintain the strip tension substantially constant during speed changes comprising a dynamo electric machine equipped with an energy storing device, means for supplying a voltage to said machine proporll tional to the speed of said element driving motor, and a coil included in circuit with said dynamo electric machine for changing the action of said current responsive element.

18. A control system for apparatus having an element operating on a strip of material and a reel for the strip comprising a motor'ior driving said reel, a motor for driving said element, a constant current regulator for said reel motor provided with an operating coil responsive to the reel motor current, and means for varying the current setting of said regulator in accordance with the acceleration of said element driving motor so as to maintain the strip tension substantially constant during speed changes comprising a pilot generator driven by said element driving motor, an auxiliary motor supplied from said generator and equipped with an energy storing fly-wheel, and a coil included in circuit with said auxiliary motor for varying the action of said operating coil.

19. A control system for mill apparatus and the like having motor driven rolls for operating on the strip and a reel for the strip, comprising a motor connected to said reel, a regulator provided with a coil responsive to the reel motor current for maintaining the reel motor current substantially constant, a rheostat included in the circuit of said coil for setting said regulator to hold a desired value of motor current, means responsive to acceleration of said mill motor for varying the setting of said regulator during acceleration of the motors comprising a dynamo electric machine influenced by acceleration of the mill motor and an auxiliary coil included in the circuit of said machine for modifying the action of said current responsive coil, and a rheostat for controlling the excitation of said auxiliary coil to compensate for changes in the effect of said regulator setting rheostat during acceleration.

20. A control system for apparatus having an element operating on a strip of material and a reel for the strip comprising a motor for driving the reel, a motor connected to said element, a regulator provided with an operating coil responsive to the reel. motor current for maintaining substantially constant current input to the reel motor, and means responsive to acceleration of said element driving motor for varying the setting of said regulator to maintain the strip tension substantially constant during acceleration and deceleration comprising an auxiliary coll on said regulator connected across the armature oi said element driving motor for varying the eii'ect 20 of said operating coil, and an auxiliary motor connected in the circuit of said auxiliary coil and provided with fiy-wheel efi ect to provide for increasing the current in said auxiliary coil during acceleration and reduction of the current in said 25 

